1. Field of the Invention
The present invention relates to optical encoders and more particularly to an apparatus for optical encoding which minimizes power consumption and is particularly adaptable for use with cursor control devices for computer controlled display systems.
2. Description of the Related Art
In many computer controlled display systems, it is desirable to allow the user to control the position of a cursor or the like by means which are external from the main computer keyboard. For example, a user may be required to repetitively choose software options displayed on a cathode ray tube (CRT), or may desire to input data in a diagram format into the computer system. In such situations, traditional keyboard input systems are not as effective as cursor control devices each commonly referred to as a "mouse" or a "trackball".
Some cursor control devices presently used have contacting encoders with radially extending conducting arms attached to an encoding wheel. The arms contact stationary brushes for closing an electrical circuit in response to the cursor control device movement.
However, most prior art methods for detecting movement of the cursor control device include optical encoding means. Such optical encoders minimize the amount of mechanical movement and contact between mechanical parts and are therefore, in that aspect, less vulnerable to wear.
Examples of prior art cursor control devices utilizing optical encoding means are provided in U.S. Pat. No. Re. 32,633, issued to D. Hovey, entitled "Cursor Control Device" and U.S. Pat. No. Re. 32,632, issued to W. D. Atkinson, entitled "Display System". Both the '632 and '633 patents are assigned to the present assignee.
Most optical encoders presently in use utilize at least one light emitting diode (LED) in conjunction with two open collector phototransistors, the LEDs transmitting photons to the bases of the phototransistors, allowing the phototransistors to conduct. Typically, an encoding disc (i.e. aperture wheel) is provided with radially spaced slits to alternately provide or interrupt passage of light to the phototransistors as the encoding disc moves in association with movement of the cursor control device. Thus, a scheme is created to either provide the dual phototransistors in dual off states, off-on states, on-off states or dual on states. The variations in these states provide indications of movement of the cursor control device. The LEDs utilized in the prior art methods are in a constant on-state during utilization of the cursor control device, resulting in constant power consumption.
With the advent of more portable computers, such as laptop computers, the need for cursor control devices which require lower power consumption has become acute. As will be disclosed below, present applicant has developed a novel apparatus and method for minimizing power consumption in such cursor control devices, with general applications where optical encoding is required.